Male chickens definitely play favorites. He may not be breeding that hen. Do you have a small enclosure where you can put just two of them? So if she is avoiding the male, she will have a harder time doing so? Say a 4' X 4" or 4' x 8' pen? Or even a huge dog crate, as long as it is high so the rooster can mate? You can try the food coloring trick. I've never tried it myself, but supposedly if you put some food coloring in the hen's vent, for the next day or two, the coloring will show up on the egg shell. A way to see which is laying. You can also move each hen in turn into a small pen until they lay an egg to see which one is laying tan. If you want to improve on egg color on your splash Marans, then why bother hatching from those tan eggs anyway? It is recommended to hatch only the darkest eggs to improve egg color. If they were my birds, I'd identify which hen is laying the light eggs and move her into a laying pen and not use her for breeding.